#include <stdio.h>
#include <string.h>
#include <stdint.h>

// MD5 Constants
#define F(x, y, z) ((x & y) | (~x & z))
#define G(x, y, z) ((x & z) | (y & ~z))
#define H(x, y, z) (x ^ y ^ z)
#define I(x, y, z) (y ^ (x | ~z))
#define ROTATE_LEFT(x, n) ((x << n) | (x >> (32-n)))

#define S11 7
#define S12 12
#define S13 17
#define S14 22
#define S21 5
#define S22 9
#define S23 14
#define S24 20
#define S31 4
#define S32 11
#define S33 16
#define S34 23
#define S41 6
#define S42 10
#define S43 15
#define S44 21

#define MD5_BLOCK_SIZE 64
#define MD5_BUF_SIZE 4096

// MD5 Context Structure
typedef struct {
    uint32_t state[4];
    uint32_t count[2];
    uint8_t buffer[64];
} MD5_CTX;

// Function Prototypes
static void md5_transform(uint32_t state[4], const uint8_t block[64]);
static void md5_encode(uint8_t *output, const uint32_t *input, uint32_t len);
static void md5_decode(uint32_t *output, const uint8_t *input, uint32_t len);
void md5_init(MD5_CTX *context);
void md5_update(MD5_CTX *context, const uint8_t *input, uint32_t inputLen);
void md5_final(uint8_t digest[16], MD5_CTX *context);
int calculate_file_md5(const char *filepath, char *md5_str);

// MD5 Transform Rounds
#define FF(a, b, c, d, x, s, ac) { \
    a += F(b,c,d) + x + ac; \
    a = ROTATE_LEFT(a,s); \
    a += b; \
}
#define GG(a, b, c, d, x, s, ac) { \
    a += G(b,c,d) + x + ac; \
    a = ROTATE_LEFT(a,s); \
    a += b; \
}
#define HH(a, b, c, d, x, s, ac) { \
    a += H(b,c,d) + x + ac; \
    a = ROTATE_LEFT(a,s); \
    a += b; \
}
#define II(a, b, c, d, x, s, ac) { \
    a += I(b,c,d) + x + ac; \
    a = ROTATE_LEFT(a,s); \
    a += b; \
}

// Initialize MD5 Context
void md5_init(MD5_CTX *context) {
    context->count[0] = 0;
    context->count[1] = 0;
    context->state[0] = 0x67452301;
    context->state[1] = 0xEFCDAB89;
    context->state[2] = 0x98BADCFE;
    context->state[3] = 0x10325476;
}

// Update MD5 Context
void md5_update(MD5_CTX *context, const uint8_t *input, uint32_t inputLen) {
    uint32_t i, index, partLen;
    index = (context->count[0] >> 3) & 0x3F;
    if ((context->count[0] += ((uint32_t)inputLen << 3)) < ((uint32_t)inputLen << 3))
        context->count[1]++;
    context->count[1] += ((uint32_t)inputLen >> 29);
    partLen = 64 - index;
    if (inputLen >= partLen) {
        memcpy(&context->buffer[index], input, partLen);
        md5_transform(context->state, context->buffer);
        for (i = partLen; i + 63 < inputLen; i += 64)
            md5_transform(context->state, &input[i]);
        index = 0;
    } else {
        i = 0;
    }
    memcpy(&context->buffer[index], &input[i], inputLen-i);
}

// Finalize MD5 Calculation
void md5_final(uint8_t digest[16], MD5_CTX *context) {
    uint8_t bits[8];
    uint32_t index, padLen;
    static uint8_t PADDING[64] = {
        0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
    };
    md5_encode(bits, context->count, 8);
    index = (context->count[0] >> 3) & 0x3f;
    padLen = (index < 56) ? (56 - index) : (120 - index);
    md5_update(context, PADDING, padLen);
    md5_update(context, bits, 8);
    md5_encode(digest, context->state, 16);
}

// MD5 Basic Transformation
static void md5_transform(uint32_t state[4], const uint8_t block[64]) {
    uint32_t a = state[0], b = state[1], c = state[2], d = state[3];
    uint32_t x[16];

    md5_decode(x, block, 64);

    /* Round 1 */
    FF(a, b, c, d, x[ 0], S11, 0xd76aa478);
    FF(d, a, b, c, x[ 1], S12, 0xe8c7b756);
    FF(c, d, a, b, x[ 2], S13, 0x242070db);
    FF(b, c, d, a, x[ 3], S14, 0xc1bdceee);
    FF(a, b, c, d, x[ 4], S11, 0xf57c0faf);
    FF(d, a, b, c, x[ 5], S12, 0x4787c62a);
    FF(c, d, a, b, x[ 6], S13, 0xa8304613);
    FF(b, c, d, a, x[ 7], S14, 0xfd469501);
    FF(a, b, c, d, x[ 8], S11, 0x698098d8);
    FF(d, a, b, c, x[ 9], S12, 0x8b44f7af);
    FF(c, d, a, b, x[10], S13, 0xffff5bb1);
    FF(b, c, d, a, x[11], S14, 0x895cd7be);
    FF(a, b, c, d, x[12], S11, 0x6b901122);
    FF(d, a, b, c, x[13], S12, 0xfd987193);
    FF(c, d, a, b, x[14], S13, 0xa679438e);
    FF(b, c, d, a, x[15], S14, 0x49b40821);

    /* Round 2 */
    GG(a, b, c, d, x[ 1], S21, 0xf61e2562);
    GG(d, a, b, c, x[ 6], S22, 0xc040b340);
    GG(c, d, a, b, x[11], S23, 0x265e5a51);
    GG(b, c, d, a, x[ 0], S24, 0xe9b6c7aa);
    GG(a, b, c, d, x[ 5], S21, 0xd62f105d);
    GG(d, a, b, c, x[10], S22,  0x2441453);
    GG(c, d, a, b, x[15], S23, 0xd8a1e681);
    GG(b, c, d, a, x[ 4], S24, 0xe7d3fbc8);
    GG(a, b, c, d, x[ 9], S21, 0x21e1cde6);
    GG(d, a, b, c, x[14], S22, 0xc33707d6);
    GG(c, d, a, b, x[ 3], S23, 0xf4d50d87);
    GG(b, c, d, a, x[ 8], S24, 0x455a14ed);
    GG(a, b, c, d, x[13], S21, 0xa9e3e905);
    GG(d, a, b, c, x[ 2], S22, 0xfcefa3f8);
    GG(c, d, a, b, x[ 7], S23, 0x676f02d9);
    GG(b, c, d, a, x[12], S24, 0x8d2a4c8a);

    /* Round 3 */
    HH(a, b, c, d, x[ 5], S31, 0xfffa3942);
    HH(d, a, b, c, x[ 8], S32, 0x8771f681);
    HH(c, d, a, b, x[11], S33, 0x6d9d6122);
    HH(b, c, d, a, x[14], S34, 0xfde5380c);
    HH(a, b, c, d, x[ 1], S31, 0xa4beea44);
    HH(d, a, b, c, x[ 4], S32, 0x4bdecfa9);
    HH(c, d, a, b, x[ 7], S33, 0xf6bb4b60);
    HH(b, c, d, a, x[10], S34, 0xbebfbc70);
    HH(a, b, c, d, x[13], S31, 0x289b7ec6);
    HH(d, a, b, c, x[ 0], S32, 0xeaa127fa);
    HH(c, d, a, b, x[ 3], S33, 0xd4ef3085);
    HH(b, c, d, a, x[ 6], S34,  0x4881d05);
    HH(a, b, c, d, x[ 9], S31, 0xd9d4d039);
    HH(d, a, b, c, x[12], S32, 0xe6db99e5);
    HH(c, d, a, b, x[15], S33, 0x1fa27cf8);
    HH(b, c, d, a, x[ 2], S34, 0xc4ac5665);

    /* Round 4 */
    II(a, b, c, d, x[ 0], S41, 0xf4292244);
    II(d, a, b, c, x[ 7], S42, 0x432aff97);
    II(c, d, a, b, x[14], S43, 0xab9423a7);
    II(b, c, d, a, x[ 5], S44, 0xfc93a039);
    II(a, b, c, d, x[12], S41, 0x655b59c3);
    II(d, a, b, c, x[ 3], S42, 0x8f0ccc92);
    II(c, d, a, b, x[10], S43, 0xffeff47d);
    II(b, c, d, a, x[ 1], S44, 0x85845dd1);
    II(a, b, c, d, x[ 8], S41, 0x6fa87e4f);
    II(d, a, b, c, x[15], S42, 0xfe2ce6e0);
    II(c, d, a, b, x[ 6], S43, 0xa3014314);
    II(b, c, d, a, x[13], S44, 0x4e0811a1);
    II(a, b, c, d, x[ 4], S41, 0xf7537e82);
    II(d, a, b, c, x[11], S42, 0xbd3af235);
    II(c, d, a, b, x[ 2], S43, 0x2ad7d2bb);
    II(b, c, d, a, x[ 9], S44, 0xeb86d391);

    state[0] += a;
    state[1] += b;
    state[2] += c;
    state[3] += d;
}

// Encode uint32 into uint8
static void md5_encode(uint8_t *output, const uint32_t *input, uint32_t len) {
    uint32_t i, j;
    for (i = 0, j = 0; j < len; i++, j += 4) {
        output[j] = (uint8_t)(input[i] & 0xff);
        output[j+1] = (uint8_t)((input[i] >> 8) & 0xff);
        output[j+2] = (uint8_t)((input[i] >> 16) & 0xff);
        output[j+3] = (uint8_t)((input[i] >> 24) & 0xff);
    }
}

// Decode uint8 into uint32
static void md5_decode(uint32_t *output, const uint8_t *input, uint32_t len) {
    uint32_t i, j;
    for (i = 0, j = 0; j < len; i++, j += 4)
        output[i] = ((uint32_t)input[j]) | (((uint32_t)input[j+1]) << 8) |
                   (((uint32_t)input[j+2]) << 16) | (((uint32_t)input[j+3]) << 24);
}

// Calculate file MD5
int calculate_file_md5(const char *filepath, char *md5_str) {
    FILE *file = fopen(filepath, "rb");
    if (!file) {
        printf("Failed to open file for MD5 calculation\n");
        return -1;
    }

    unsigned char buffer[MD5_BUF_SIZE];
    unsigned char digest[16];
    size_t bytes_read;
    MD5_CTX md5_ctx;

    md5_init(&md5_ctx);

    while ((bytes_read = fread(buffer, 1, sizeof(buffer), file)) > 0) {
        md5_update(&md5_ctx, buffer, bytes_read);
    }

    md5_final(digest, &md5_ctx);
    fclose(file);

    // Convert to hex string
    for (int i = 0; i < 16; i++) {
        sprintf(&md5_str[i * 2], "%02x", digest[i]);
    }
    md5_str[32] = '\0';

    return 0;
}